1. Field of the Invention
This invention relates to a water gun for use as a seismic acoustic source at sea, wherein a high-velocity water jet is injected into the surrounding water. The momentum of the water jet creates a cavity in the surrounding water which implodes to create an acoustic wave.
2. Description of the Prior Art
In general water guns used for seismic exploration consist of a housing having at least one closed end. A reciprocally sliding piston divides the housing into two parts: A closed firing or compression chamber and a water-charge chamber, which may or may not be free-flooding, for containing a volume of water. In use, a restoring force causes the piston to retract into the compression chamber against a pneumatic or hydraulic spring system, where the piston is releasably locked into place. Upon command, the piston is released. Under the urging of the spring system, the piston impulsively drives the volume of water from the water-charge chamber as a jet-like water slug. The ejected water slug creates a momentary cavity in the surrounding water. A sharp acoustic pulse is generated when the cavity collapses.
There are a number of patents for water guns, all of which are based upon the principle above described. Variations in the known design of water guns are directed towards improvements in the acoustic signature of the gun and towards mechanical simplicity and reliability. Typical patents will now be discussed.
U.S. Pat. No. 4,131,178 describes a water gun that consists of a cylindrical housing closed at both ends. A sliding piston divides the interior of the housing into two chambers. One chamber, the air chamber, receives air under pressure. The other chamber, the water chamber, receives a volume of water having a pressure greater than the pressurized air. The housing includes an exhaust port that communicates with the water chamber; the exhaust port is closed by a slidable sleeve valve. In operation, with the sleeve valve closed, pressurized water is forced into the water chamber, driving the piston towards one end of the housing to further compress the air in the air chamber. Upon command, the sleeve valve opens the exhaust port, allowing the pressurized water to begin to escape, thereby reducing the pressure working against the piston. The air pressure in the air chamber therefore causes the piston to abruptly eject the water from the water chamber to create the desired water jet. I have found by observation that the gun in U.S. Pat. No. 4,131,178 appears to have certain serious mechanical problems, such as shuttle binding, leakage around the sleeve valve and inadequate shuttle damping.
U.S. Pat. No. 4,185,714 teaches a water gun having two differential-area pistons connected by a piston shaft to form a piston assembly. The piston assembly is set and triggered pneumatically by a set of valving devices in an upper chamber. The lower portion of the gun housing includes a free flooding water chamber which is sealed off from the upper chamber by the lower piston of the piston assembly. When triggered, the piston assembly is abruptly driven downwardly by an air spring in the upper chamber, thereby forcibly ejecting the slug of water contained in the lower portion of the housing. One of the drawbacks to that gun is its mechanical complexity as well as its inherent operational sluggishness.
U.S. Pat. No. 4,234,052 like U.S. Pat. No. 4,131,178, employs a slug of pressurized water to provide a means for recharging an air spring in an air chamber. In operation, gas under pressure is applied to an operating chamber against a reciprocal shuttle to cause the shuttle to close a charge chamber having a discharge port. Water is pumped into the charge chamber to push a piston follower into an energized piston against a pneumatic spring in an air chamber for recharging the penumatic spring. When the air pressure on opposite sides of the operating piston is equalized, the piston follower is driven towards the discharge port to eject the water therefrom. Unlike U.S. Pat. No. 4,131,178, the pneumatic springs form a closed system.
U.S. Pat. No. 4,303,141 provides a water gun consisting of a housing having a discharge port that is in communication with the surrounding water. A first and a second shuttle are slidingly mounted inside the main housing. The first shuttle in combination with the housing forms a water chamber for containing a volume of water. A force producing means cyclically cause the shuttles to move away relative to each other, or in locked condition with each other so that by applying an brupt propulsive source during each cycle of operation, the contained volume of water is expelled through the discharge port.
The latter two water guns are very complex and, because of their physical size, are difficult to handle. I have found, through actual observation of many existing water guns that the moving parts, pistons, and shuttles are inadequately damped at the end of an operating stroke. Many of the known guns self-destruct under the large forces that develop during operation.
It is an object of this invention to provide a simple, compact water gun that operates as a closed system to avoid the necessity for excessive replenishment of pressurized operating fluids. Further objects are to provide for rapid cycling of the gun by using relatively light-weight parts and to provide adequate cushioning for those moving parts that must intimately interact with each other during operation.